Optical storage devices use information carriers with high density of recording, which are written and read out by focussed laser beams. A promising technique for rewritable optical data recording is magnetooptic storage, in particular, magnetooptic disks with random-access capability. In magnetooptic storage media, data bits are written into concentric or spiral tracks by laser heating tiny spots of a thin, axially-magnetized layer on a disk substrate above the so-called Curie temperature. Upon cooling in the presence of an external magnetic field, which is oriented opposite to the axial magnetization, the heated spots will reverse their magnetization direction and form information indicia with dimensions in the micrometer range.
The recorded information is retrieved by reflecting a focussed, low-power laser beam on the track and sensing the rotation of the plane of polarization in the reflected light, which occurs when the laser beam passes through a spot of reversed magnetization. The angles of rotation in this "magnetooptic-Kerr effect" are very small and in the order of degrees of arc. A similar technique for reading out magnetooptic media uses the Faraday effect in light that is transmitted through a transparent magnetooptic layer.
In addition to extracting data information, the analysis of the reflected light beam must yield servo control signals for maintaining the readout beam focussed and locked in the tracks of data bits. In conventional devices for magnetooptic storage, this analysis was performed by an optical system with a considerable number of high-quality components that are bulky, expensive and difficult to adjust. Examples of such optical beam analyzing systems can be found, e.g., in EP-A-No. 156460 or DE-A-No. 3334120.
EP-A-No. 226647 discloses a read/write head for an optical disk, which avoids individual optical components and integrates the beam-analyzing function (with beam splitter, beam-shaping elements and photodetectors) into the light-guiding access arm for the optical disk. This integrated access arm is, however, difficult to manufacture and does not yet achieve an optimum mass distribution of the access arm which is required for high-access speed to selected data tracks.